As primers for improving the coatability characteristics (film-forming properties, adhesion, etc.) to polyolefin plastics products such as automobile interior and exterior members, there are known in the art fully aqueous primers combining a carboxy-modified polyolefin resin, a hydroxy-modified polyolefin resin and a polystyrene-poly-conjugated diene-polystyrene triblock copolymer (cf. e.g. Japanese Kokai Publication Hei-11-29673), and primers capable of being rendered completely aqueous combining an aqueous dispersion of a polyolefin resin, an aqueous dispersion of an acrylic resin and an aqueous dispersion of a urethane resin (cf. e.g. Japanese Kokai Publication Hei-06-336568).